Cryptography and SSL/TLS Toolkit



RAND_bytes, RAND_priv_bytes, RAND_bytes_ex, RAND_priv_bytes_ex, RAND_pseudo_bytes - generate random data


 #include <openssl/rand.h>

 int RAND_bytes(unsigned char *buf, int num);
 int RAND_priv_bytes(unsigned char *buf, int num);

 int RAND_bytes_ex(OPENSSL_CTX *ctx, unsigned char *buf, int num);
 int RAND_priv_bytes_ex(OPENSSL_CTX *ctx, unsigned char *buf, int num);

Deprecated since OpenSSL 1.1.0, can be hidden entirely by defining OPENSSL_API_COMPAT with a suitable version value, see openssl_user_macros(7):

 int RAND_pseudo_bytes(unsigned char *buf, int num);


RAND_bytes() puts num cryptographically strong pseudo-random bytes into buf.

RAND_priv_bytes() has the same semantics as RAND_bytes(). It is intended to be used for generating values that should remain private. If using the default RAND_METHOD, this function uses a separate "private" PRNG instance so that a compromise of the "public" PRNG instance will not affect the secrecy of these private values, as described in RAND(7) and RAND_DRBG(7).

RAND_bytes_ex() and RAND_priv_bytes_ex() are the same as RAND_bytes() and RAND_priv_bytes() except that they both take an additional ctx parameter. The DRBG used for the operation is the public or private DRBG associated with the specified ctx. The parameter can be NULL, in which case the default library context is used (see OPENSSL_CTX(3). If the default RAND_METHOD has been changed then for compatibility reasons the RAND_METHOD will be used in preference and the DRBG of the library context ignored.


Always check the error return value of RAND_bytes() and RAND_priv_bytes() and do not take randomness for granted: an error occurs if the CSPRNG has not been seeded with enough randomness to ensure an unpredictable byte sequence.


RAND_bytes() and RAND_priv_bytes() return 1 on success, -1 if not supported by the current RAND method, or 0 on other failure. The error code can be obtained by ERR_get_error(3).


RAND_add(3), RAND_bytes(3), RAND_priv_bytes(3), ERR_get_error(3), RAND(7), RAND_DRBG(7)


  • RAND_pseudo_bytes() was deprecated in OpenSSL 1.1.0; use RAND_bytes() instead.

  • The RAND_priv_bytes() function was added in OpenSSL 1.1.1.

  • The RAND_bytes_ex() and RAND_priv_bytes_ex() functions were added in OpenSSL 3.0

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